The Age And Tectonic Evolution Of The Dongga Instrusive Of Gangdese In North Of Shigatse

Qinghai-Tibet Plateau has long been one of the most focusing area in bothdomestic and foriegn geology research due to its distinct lithosphere structure,tectonic evolution and tectonic characteristics. But the research work carried out therehave been limited because the severe working condition both in position andenvironment. Therefore, as an important part of the history and structure inQinghai-Tibet Plateau, the tectonic setting and tectonic evolution of Gangdese aredefinitely meaningful to the research in the evolutional history of Qinghai-TibetPlateau.This paper discusses the igneous rock of Dongga gully in the Dongga villagelocating in the northern area of Shigatse city, which is a part of south rim of middleGangdese. Because there is a wide range of discrepancy about the timing ofNeo-Tethyan initiation and Neo-Tethyan subduction, it is necessary to survey the LatePaleozoic-Mesozoic igneous rock. However, the work in southern segment ofGangdese has been focused on the area in Qushui rock,Sangri volcanic magmaticarc,etc.,and the Cretaceous and Paleogene igneous rock. There are rarely researchabout Early Jurassic igneous rock and long-time controversy about its reason andsetting: some insist on that it is the result of the Neo-Tethys oceanic crust subductednorthward; other insist on that it is the result of Bangong Co-Nujiang oceanic crustsubducted southward. The igneous rock of Dongga gully are mainlyintermediate-basic rocks, including gabbro, gabbro diorite, tonalite, oblique longgranite and diabase. Additionally, there are a mass of mafic microgranularenclaves(MME) in each unit; the geochemical data indicate that the volcanic rockshere are mainly intermediate-basic rocks of medium-K calc-alkaline series and bothsame in trend and source in terms of the host-rocks and MME. The U-Pb ages hereare mainly Early Jurassic(178Ma), with aging from north to south.Since the new evidence of Neo-Tethys oceanic basin in Late Triassic and thelocation is near the Neo-Tethys oceanic crust much more than the Bangong Co-Nujiang oceanic crust, combining the regional geological survey, the data indicatingthat the igneous rock of Dongga gully should be the result Neo-Tethys oceanic crustsubducted northward; and the time of Neo-Tethyan subduction should be earlier atleast than Early Jurassic.